Switching regulator

ABSTRACT

A switching regulator for generating a regulated output voltage comprises a charging switch, a network with a coil, a capacitor and a diode as well as a control to open and close the charging switch. On the output side, the network has a positive output rail and a negative output rail, wherein the output voltage to be regulated is applied between the positive output rail and the negative output rail. The control is formed to control a change-over switch to connect the control input of the charging switch either to the positive output rail or the negative output rail via a potential coupling means. Thus, for controlling the switch, no individual potential has to be generated, but the potential at the positive output rail or the negative output rail, which is anyway applied, is used for controlling the charging switch. Thereby, the inventive switching regulator can be structured in a simple way and, additionally, provides defined and fast startup without requiring additional startup circuits.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of copending International Application No. PCT/EP2004/009497, filed Aug. 25, 2004, which designated the United States, was not published in English, is incorporated herein by reference in its entirety, and which claimed priority to German Patent Application No. 10339483.4, filed on Aug. 27, 2003, and to German Patent Application No. 102004004542.9, filed on Jan. 29, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching regulator and particularly to switching regulators with a network of a coil, a capacitance and a diode, which are connected in a ring-like way.

2. Description of the Related Art

FIG. 4 shows a known down converter with a simple switch, such as, for example, shown in “Halbleiter-schaltungstechnik” U. Tietze, C. H. Schenk, Springer-Verlag, 9. Auflage, 1989, illustration 18.37 on page 564. The down converter in FIG. 4 comprises a ring-like circuit with a coil 400, a capacitor 402 and a diode 404. Further, the down converter or switching regulator, respectively, in FIG. 4 comprises a charging switch 406 as well as a control not shown in FIG. 4, which is formed to control the charging switch 406 such that the output voltage of the switching regulator, indicated by U_(SR) in FIG. 4, is maintained on a defined level or in a range around the defined level, respectively.

The circuit shown in FIG. 4 comprises a determined number of nodes, which will be discussed below. A pole of an input voltage source U₀ is connected to a first input node 410 of the circuit, while another potential of the input source U₀ is connected to a second input node 412. The second input node 412 is typically the ground node. A first output node 414 is also referred to as first output rail or positive output rail, while a second output node 416 is also referred to as second output rail or negative output rail, respectively, when the convention shown in FIG. 4 is used for the output voltage of the switching regulator U_(SR). On the one hand, the switch 406 is connected between the first input node 410 and a first intermediate node 418. Further, the diode 404 is connected between the first intermediate node 418 and the second input node 412, such that the anode of the diode is connected to the second input node 412, while the cathode of the diode is connected to the first input node 418. Further, as shown in FIG. 4, the capacitor 402 is connected between the first output node 414 and the second output node 416. According to the configuration of the network of diode, coil and capacitor shown in FIG. 4, the coil is connected between the first intermediate node 418 and the first output node 414.

Below, the functionality of the circuit shown in FIG. 4 will be discussed. As long as the switch 406 is closed, U_(D) is equal to the negative input voltage U₀. If it opens, the inductor current I_(L) maintains its direction, and the amount of U_(D) sinks, until the diode becomes conductive, which means to about 0 potential. The time curve of the coil current results from the law of induction, according to which the voltage at the coil is equal to the product of inductance L of the coil and the derivation of the coil current according to time. During the switch-on time, which means when the input voltage U₀ is applied to the diode 404, the voltage U₀−U_(SR) is applied to the inductor. During the switch-off time t_(aus) of the switch 406, the voltage U_(L)=−U_(SR) is applied to the inductor. Therefore, an amount of current change ΔI_(L) results, which is as follows: ΔI _(L)=1/L·(−U _(SR))·Δt _(aus)=1/L(U ₀ −U _(SR))t _(ein) From this balance, the output voltage can be calculated again, which is defined as follows: U _(SR) =[t _(ein)/(t _(ein) +t _(aus))]·U ₀ =t _(ein) /T·U ₀ =p·U ₀. In the previous equation, T=t_(ein)+t_(aus)=1/F is the oscillation period and p=t_(ein)/T is the so called duty cycle. As expected, it can be seen that the arithmetic average of U_(D) results as output voltage. Typically, the inductance L of the coil 400 is chosen such that the minimum current is not undershot, as is known in the art. Further, it is known that by increasing the clock frequency, the inductance can be reduced. Further, with too high frequencies, the effort for the switching transistor and the control circuit increases. Additionally, dynamic switching losses increase in proportion to the frequency.

The capacitor 402 and the inductor 400 determine the ripple of the output voltage. Generating the switching signal for switching the charging transistor 406 is usually performed by a pulse width modulator and a regulator with voltage reference. In particular, a reference voltage providing a set value is supplied to a subtracter, to which the current output voltage U_(SR) is also supplied as actual value. The output signal of the subtracter is supplied to a variable gain amplifier, feeding a comparator, to which, on the other hand, a signal generated by a saw tooth generator is supplied. The output signal of the comparator is the control signal for the switch 406 in FIG. 4. The variable gain amplifier is typically a PI variable gain amplifier. The same increases its output signal for so long until the difference at the output of the subtracter becomes 0, which means until the output voltage U_(SR) is equal to the set output voltage. Typical ranges for dimensioning the coil are in the millihenry range (e.g. 2.7 mH), while typical values for capacitors are in the three-digit micro Farad range (e.g. 100 μF), when switching frequencies in the range of 50 kHz are used.

Switching regulators shown in FIG. 4 are to provide a suitable voltage supply to a subsequently connected circuit, such as an ASIC. The voltage supply consists normally of one or several constant direct voltages of, for example, +5 V or ±15 V. Frequently, the same is not available in the desired form from the start and has to be generated first by, for example, a switching regulator shown in FIG. 8 that can be supplemented by a downstream linear regulator to remove the ripples of the output voltage. Usually, a rectifier is at the input side of the switching regulator shown in FIG. 8 which generates the input voltage U₀ from the alternating current or three-phase current net (230 V or 400 V), respectively, of the power station.

Thus, in deviation from the switching regulator shown in FIG. 4, other regulators exist, with a transformer, a rectifier, a smoothing capacitor and possibly a linear regulator for voltage stabilization. However, the transformer is difficult to produce and thus expensive. Further, it requires a lot of space. A further disadvantage of the transformer is its frequency-depending working range. This is, for example, limited to the network frequency of 50 Hz or 60 Hz, respectively. If the frequency deviates, this also causes a deviation of the output voltage of the transformer. The voltage transfer does not work with a direct voltage at the input.

If the transformer is omitted and only rectifier, smoothing capacitor and linear regulator are used, a lot of energy in the form of heat is lost. Additionally, sufficient cooling of the linear regulator has to be provided, which is again very expensive and requires space. All this is avoided by using a switching regulator instead of the linear regulator, as illustrated with regard to FIG. 4. By the significantly better efficiency, little energy in the form of heat is lost and thus the effort for the cooling is significantly lower. As has already been discussed, the switching regulator requires an inductor (the inductance 400 in FIG. 4), which is relatively expensive in the production. However, the same has only one winding and is thus simpler to produce than a transformer having two windings. Above that, the inductor can be minimized by selecting a higher operating frequency, which also works for transformers.

Many known switching network parts, such as the switching network part shown in FIG. 4, are problematical in some regard. Normally, the input voltage range is limited to a ratio of U_(E,max)/U_(E,min)≦5, which can be seen from catalogs of different providers. This range is too low for some applications and should be increased to a ratio of about 20:1.

The voltage supply of the regulator itself is either performed via a separate voltage source or is generated from the input voltage, which means an additional voltage regulator and thus additional effort.

Further, for a flexible usage, it is intended to be able to select the input voltage significantly higher than the maximum allowable operating voltage of the regulator itself, without using additional voltage regulators for generating this operating voltage.

Additionally, when applying the input voltage, a possibly fast controlled starting of the switching regulator should be ensured. This so called starting-delay should be as small as possible, particularly for time-critical applications.

It is a particular disadvantage of the switching regulator concept shown in FIG. 4 that the signal for controlling the switch 406 has to be generated externally. Thus, on the one hand, the timing of the signal is determined by regulating the output voltage U_(SR) by an externally supplied regulator. Further, the signal actually applied to the switch, which can, for example, be a transistor, which will be the gate voltage of the transistor in the case of the transistor, is again generated externally in a switching regulator, which again results in a voltage converter, to apply a control signal with the correct potential to the switch. As has been discussed, the signal directly applied to the switch 406 is thus either provided externally or generated from the input voltage U₀.

DE 199 46 025, DE 197 00 100 C2, DE 195 07 553 A1 or DE 197 06 491 A1 disclose switching regulators operating based on the basic block diagram shown in FIG. 4, wherein depending on the embodiment, the coil 400 is either connected between the first intermediate node 418 and the first output node 414, as shown in FIG. 4, or alternatively, between the second input node 412 and the second output node 416. The control voltage for the switch 406, which can be designed as transistor, is determined by the regulator determining the timing of the switch 406 or generated externally. This causes additional switching effort, which means additional costs with regard to design, testing and production. Additional costs are particular disadvantageous since switching regulators, particularly when they are provided within lamps or also designed fully integrated with an integrated circuit to be supplied, increase the price of the final product and particularly occur in chip applications where the chip area is a criterion, due to the increased chip area consumption.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a switching regulator concept which is less expensive to produce.

In accordance with a first aspect, the present invention provides a switching regulator for generating a regulated output voltage, having: a first output rail; a second output rail; wherein one output rail can be brought to a negative potential and the other output rail to a positive potential; a diode connected to an output rail; a charging switch connected in series to the diode, wherein the charging switch has a control input by which the charging switch can be opened or closed; a capacitor, wherein the regulated output voltage can be tapped at the capacitor; an inductance coupled to the diode; a change-over means controllable to couple the first output rail or the second output rail to the control input of the charging switch in dependence on a change-over control signal, wherein the charging switch is formed to be closed due to a potential at one output rail and to be opened due to a potential at the other output rail; and a control for providing the change-over control signal to close the charging switch, so that the capacitor is charged, and to open the charging switch, so that the capacitor is discharged, so that the output voltage can be regulated.

In accordance with a second aspect, the present invention provides a method for controlling starting up of a switching regulator with a first output rail; a second output rail, wherein one output rail can be brought to a negative potential and the other output rail to a positive potential; a diode connected to an output rail; a charging switch connected in series to the diode, wherein the charging switch has a control input by which the charging switch can be opened or closed; a capacitor, wherein the regulated output voltage can be tapped at the capacitor; an inductance coupled to the diode; and a change-over means controllable to couple the first output rail or the second output rail to the control input of the charging switch in dependence on a change-over control signal, wherein the charging switch is formed to be closed due to a potential at one output rail and to be opened due to a potential at the other output rail, and a control for providing the change-over control signal to close the charging switch, so that the capacitor is charged, and to open the charging switch, so that the capacitor is to be discharged, having the steps of: applying a direct voltage between an anode of the diode and a terminal of the charging switch, which is not connected to a cathode of the diode; supplying the control with the output voltage between the first output rail and the second output rail; and controlling the change-over switch by the control means, when the output voltage has reached a value where the control can operate in an determined way, to connect the change-over switch to the output rail, which is coupled to the cathode of the diode.

The present invention is based on the knowledge that the signal for controlling the switch is derived from the output signal of the switching regulator,. which means from the useful signal generated by the switching regulator. Herefore, according to the invention, the potential is tapped off at an output rail and supplied to the switch either directly or provided with a voltage drop to close the switch, for example. If the switch is to be opened, this is performed by applying the potential at the other output rail, which means, for example, the negative output rail, either directly or provided with a voltage drop to the control input of the switch. Changing-over from one output rail to the other output rail is performed, according to the invention, by a change-over switch with a control input controlled by the timing signal of the control for determining the opening and closing times of the charging switch.

The present invention is advantageous in that the control determining the switch timing does not have to generate a specific voltage with a specific voltage value to open the switch on the one hand or to close the same on the other hand. Further, no such potential has to be generated externally. Instead, according to the invention, the potentials on the first and second output rail, which are anyway present in the switching regulator, are used to either open or close the switch with a potential differing from the potential at the other output rail by the output voltage to be regulated. Since the potential difference of these two potentials is typically high enough, no particularly expensive dimensioning of a transistor has to be used, specially when a transistor is used as switch, since it is sufficient when the transistor is designed such that the same closes with the one potential at its control input and is opened with the other potential at its control input. As has been discussed, since the potential difference between the potentials at the two output rails will be typically in the range of several volt, a transistor switch does not have to be specially dimensioned, since a lot of transistors exist, which are conducting with one potential at the control input and which are not conducting with the other potential at the control input.

It is another advantage of the present invention that no external circuit, which again causes current consumption, is required, since the energy for controlling the switch is directly drawn from the switching regulator output voltage. Compared to the known case, where the energy for controlling the switch is drawn from the input voltage, the inventive variation is advantageous in that the switching regulator output voltage is almost always smaller than the input voltage, wherein smaller voltages can be handled all in all with less expensive elements than higher voltages, which is particularly the case for the change-over switch between the one and the other output rail.

The present invention is further advantageous in that it does not influence the inner concept of the switching regulator, which means the cooperation of diode, coil and charging capacitor or filter capacitor, respectively, in that the functionality had to be altered to provide a known switching regulator principle with the inventive change-over switch. Instead, the inventive concept for generating the switching signal for the charging switch is compatible with known switching regulator concepts, which again saves time and costs, since known concepts can be used.

It is a further advantage of the present invention that the inventive concept can be combined with a self-supplying control, since it requires no supply of its own. Thus, when starting up the switching regulator, i.e. when switching on the input voltage, it is at first insignificant in which position the change-over switch is, for the startup process and up to a certain threshold determined by the charging switch. The inventive switching regulator will always start up, independent of whether the change-over switch is connected to the positive or the negative output rail at the time of applying the input voltage. Thus, no measures have to be taken for starting up with regard to the change-over switch for applying the potential of an output rail at the control input of the switch,. Instead, the inventive switching regulator always starts up in a defined way.

A further advantage resulting from the secure startup ability is that fast starting up is assured, particularly since no time delay occurs, for example due to the fact that the change-over switch has to be brought into a certain state prior to starting up or immediately at the time of starting up, respectively.

Particularly in the variation where the coil is connected to the anode of the diode, the present invention is advantageous in that the input voltage of the circuit is only limited by the electric strength of the three elements switch, freewheeling diode and coil. Further, the switching regulator supplies itself and starts up in a defined way, as has been discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will become clear from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a basic block diagram of the inventive switching regulator;

FIG. 2 a is a preferred embodiment of the inventive switching regulator with a normally-off transistor as charging switch;

FIG. 2 b is alternatives for the series resistor to generate the voltage drop before the Zener diode;

FIG. 3 a is an alternative preferred embodiment of the present invention with a normally-on transistor as charging switch;

FIG. 3 b is an alternative embodiment with a PMOS transistor as switch; and

FIG. 4 is a block diagram of a known switching regulator also known as buck converter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an inventive switching regulator for generating a regulated output voltage U_(SR) by using an input voltage U₀, which is shown for clarity reasons as voltage source U₀ 10, which is connected in series to an inner resistor R_(i) 12. If an on/off switch 14 is operated, a voltage is applied between a first input node 110 and a second input node 112. A controllable switch 106 and a network 108, which is a typical switching regulator network with a coil, a capacitor and a diode, are connected in series between the first input node 110 and the second input node 112. Typically, such switching regulator networks 101 also have a shunt resistor to provide a current path with a defined ohmic resistance.

The output voltage regulated by the switching regulator shown in FIG. 1 lies between a first output rail also referred to as positive output rail and identical to the first output node 114, and a second output rail also referred to as negative output rail and identical to the second output node 116. In the embodiment shown in FIG. 1, the positive output rail 114 can be brought to a first (positive) potential, while the negative output rail 116 can be brought to a second potential, which is smaller than the first potential.

The network 101 having a coil, a capacitor and a diode, has the diode particularly switched such that it is coupled to the positive output rail when the coil is coupled to the negative output rail, or that it is coupled to the negative output rail when the coil is coupled to the positive output rail. Typically, the diode will thus be coupled to an output rail, either the positive output rail 114 or the negative output rail 116.

The switch 106 shown in FIG. 1 is also referred to as charging switch, since it is provided for switching the capacitor in the network 101. Particularly, it is connected in series to the diode. The charging switch comprises further a control input 107, across which the charging switch 106 can be closed, which means that the input voltage does not drop at the charging switch but at the network 101. If, however, the switch is opened, the input voltage drops across the switch, the network 101 is thus not provided with the input voltage (apart from transient states).

Thus, the charging switch can be closed or opened via the control input 107. Further, the network 101 comprises a capacitor, which is also referred to as filter capacitor and is switched such that the regulated output voltage can be trapped at the capacitor. Further, a significant part of the network 101 is the inductance, which will be referred to below as coil, which is on the one hand coupled to the diode and on the other hand to the capacitor.

According to the invention, the switching regulator shown in FIG. 1 further comprises a change-over means 109, which is controllable to couple either the first output rail 114 or the second output rail 116 to the control input 107 of the charging switch via a coupling means 113 depending on a change-over control signal provided by a control 111. Particularly, the charging switch 106 is formed such that it is closed due to a potential at an output rail, and opened due to a potential at the other output rail. In embodiments discussed below, the charging switch 106, as will be discussed below with regard to FIGS. 2 and 3 a, is designed as normally-on or normally-off NMOS transistor. In that case, the charging switch 106 is closed (made-conductive) by the potential at the positive output rail, while it is opened (open-circuited) by the potential at the negative output rail.

If the switch 106 is designed as PMOS transistor, the coil 100 and possibly the shunt resistor 101 will be connected between the nodes 118 and 114, and the nodes 112 and 116 are short-circuited. The potential at the negative output rail will be used to open the switch while the potential at the positive output rail 114 is used to close the switch 106, which means to bring it into open-circuit operation. A respective circuit is shown in FIG. 3 b.

Normally, the control Ill will comprise a regulator which can be designed in any way, as long as it outputs a signal which causes the output voltage U_(SR) to have a defined desired time curve by opening and closing the charging switch 106. Normally, the regulator 111 will operate such that it ensures an output voltage with an average value at the output 114, 116 of the network 101, independent of which load is switched on or which current draws a load, respectively. The time curve of the voltage U_(SR) will typically be a voltage curve with certain ripples around an average value. The ripples can be brought within predetermined tolerance ranges by dimensioning the coil and capacitor, whereby in many cases a signal with ripples around an average value is already sufficient, particularly when the requirements of the switching regulator output signal, which means the voltage U_(SR), are not very high, such as for a lamp or something similar. On the other hand, in the case where the switching regulator as shown in FIG. 1 is integrated on a substrate together with an integrated circuit, which is to be supplied by the same, the same can also be supplemented by a linear regulator downstream of the output 114, 116 of the switching regulator, to fulfill requirements of a voltage U_(SR), which exceed the possibilities of the switching regulator shown in FIG. 1, to provide an output signal U_(SR) with corresponding specification with reasonable effort.

The switch-on/switch-off control is performed such that when the control 111 determines the charging switch is to be opened, the change-over switch 109 is connected to the output rail associated to opening the charging switch 106, in FIG. 1 for example 116, while when the charging switch 106 is to be closed because the capacitor in the network 101 has to be recharged, the control 111 activates the change-over switch 109, to now couple the potential of the upper output tail 114 to the control input 107 of the charging switch 106.

FIG. 2 a shows a preferred embodiment of the present invention, where the coil 100 is connected to the diode 104 and the filter capacitor 102. In the comparison of FIG. 2 a and FIG. 4, it becomes clear that the coil 100 is no longer connected to the cathode of the diode 104 as in FIG. 4, but with the anode of the diode 104. Further, it can be seen from the comparison of FIG. 2 a and FIG. 4, that now the positive output rail 114 with an intermediate node 118, across which the charging switch is connected to the diode 104, coincides with the output node 114, which means the first output rail. Further, a shunt resistor R_(Sh) 101 is shown in FIG. 2 a, which is connected between the coil 100 and the negative output rail 116. A node 103 between the shunt resistor 101 and the coil 100 is also referred to as lower intermediate node.

Particularly, the charging switch 106 in the embodiment shown in FIG. 2 a is designed as normally-off NMOS transistor, whose drain D is connected to the first input node 110, and whose source S is first short-circuited to a bulk terminal B of the transistor, and whose source S is further directly coupled to the positive output node 114, which means the positive output rail. In the embodiment shown in FIG. 2 a, the coupling means 113 of FIG. 1 comprises a Zener diode D_(Z) 113 a, a capacitor C_(Z) 113 b connected in parallel as well as a series resistor R_(Z) 113 c. The series resistor serves to supply the parallel circuit of the capacitor 113 b and the diode 113 a forming a voltage drop means across the first input node 110 with current, as will be discussed below.

FIG. 2 b shows alternatives for the series resistor to generate the voltage drop before the Zener diode. Particularly, instead of the series resistor, a current source, two alternatives of which are shown in the center and on the right hand side of FIG. 2 b, or any other bias source can be used.

For the mode of operation of the switching regulator shown in FIG. 2 a and switching regulators in general, respectively, the transient characteristics of the memory elements coil L and capacitor C_(s) are important, since the switching regulator as shown in FIG. 2 a achieves a regulation of the output voltage by constantly switching on and off of the charging switch 106.

A coil is characterized in that the voltage falling across the coil is equal to the time leakance of the current flowing through the coil. Correspondingly, a capacitor is characterized in that the current flowing through the capacitor is proportional to the time leakage of the voltage applied to the capacitor.

Further, with regard to the coil, it is of great significance that the voltage applied to the coil can jump, but that the current through the coil cannot jump. Thus, if a coil is switched on, which means a direct voltage (across a source inner resistor) is applied to the coil, the voltage at the coil will increase abruptly to the value of the applied direct voltage and then falls exponentially. Simultaneously, the current slowly starts to increase through the coil from its original value 0, until it reaches the value which is equal to the quotient from the applied voltage and the inner resistor of the source. When this stationary time is reached, the voltage falling across the coil has also become 0.

The same applies inversely to the capacitor. If the capacitor is switched on, the voltage will increase slowly at the capacitor, while the current through the capacitor can jump.

Thus, the elements coil and capacitor differ in that the current can jump through the capacitor while the voltage can jump at the coil. However, the voltage at the capacitor cannot jump. The same applies also to the current through the coil, it cannot jump either. This leads to the fact that the coil operates as short at the time of switching on, which means at the time t=0, when switching on takes place at the time t=0, while the capacitor operates as short.

In the following, reference will be made to switching off of the elements. If a capacitor charged to a direct voltage is separated from the direct voltage source, nothing will happen. The same holds the charge when it cannot discharge across a resistor. However, if switching-off of the capacitor takes place across a short, this causes that a very high current flows, wherein at this time the current and voltage arrow at the capacitor point to opposite directions. Thus, the capacitor operates as generator when it is switched off.

If a coil, through which current is flowing, is switched off, it is again of significance how the coil is switched off. If current is flowing through the coil, a magnetic field exists, wherein energy is stored. If a coil is separated from the source by an open between source and coil, the current through the coil would actually be brought to 0 immediately. However, the energy stored in the magnetic field of the coil has to be allowed to leak. Thus, switching off a coil by generating an open between the source and the coil leads to a high voltage peak, which leads to an arc being formed at the switch across which the energy stored in magnetic field can leak off to the source. For that reason, a coil is discharged across a resistor, a diode or a capacitor in order to avoid the theoretically infinitely high voltage peaks, which would have a destructive effect. This leads to a voltage peak with reduced height. If a coil is switched off, the voltage arrow and the current arrow again point to opposite directions. Thus, the coil operates as generator.

If, as shown in FIG. 2 a (or FIG. 3 a), the coil is connected in series to a capacitor, then the capacitor is a short and the coil an open at the time of switching on, which means when a voltage of 0 is changed to a voltage with a certain direct voltage value. Thus, the whole switch-on voltage is first applied to the coil and sinks then with increasing current through the series circuit of coil and capacitor. At the time of switching-off a voltage at the coil, the coil still generates a current, which keeps flowing for a certain time, until the energy stored in the coil (or the energy stored in the capacitor, respectively) has leaked off.

The functionality of the switching regulator in stationary state will be described below with reference to FIG. 2. At the time when the switch 106 is closed, the full voltage U_(G1) 10, 12 is applied to the diode 104. A current flows across the switch 106 and the coil 100, which charges the capacitor 102. Thus, the potential of the positive output rail 114 rises relative to the negative rail.

On average, the current through the coil 100 is equal to the current supplied to the load connected between the positive output rail 114 and the negative output rail 116.

At switch-on time, the node 118 is drawn to the potential of the node 110. Accordingly, the full input voltage U_(GL) is applied to the diode 104. Since the voltage U_(SR) across the capacitance C_(S) cannot jump, but the voltage cross the coil L, the node 116 is also drawn high, so that the voltage U_(SR) remains constant. Accordingly, the voltage U_(GL)−U_(SR) is applied to the inductor L, so that the current through the inductor increases according to the equation dI_(L)/dt=(U_(GL)−U_(SR))/L. Here, the voltage drop at Rsh has been neglected. Since after switching off the switch 106, the current of the inductor can no longer flow through the source U_(GL) and the switch 106, and the inductor avoids a jump of its current, this current has to continue to flow across the free wheeling diode 104. However, this leads to the fact that the potential of the node 118 jumps to a potential which lies about 0.7 V below the potential of the node 112 corresponding to the characteristic curve of this diode. Again, the capacitor C_(S) avoids a jump of the voltage U_(SR). Thus, at switch-off time, the potential of the node 116 jumps to −(0.7 V+U_(SR)). Consequently, a negative voltage is applied to the inductor (node 103−node 112), so that the current through the inductor 100 decreases corresponding to the equation dI_(L)/dt=(U_(SR)+0.7 V)/L.

In other words, since the full voltage is applied to the diode 104 at the time of switching-on the switch 106, and since the voltage at the capacitor cannot jump as discussed above, but can jump at the coil, the potential of the negative output rail 116 is risen simultaneously the switch-on time.

As long as the switch is off, the inductor feeds the capacitor across the free wheeling diode. As long as the inductor current is higher than the load current, the voltage at the capacitor increases. The more energy passes from the inductor to the capacitor, the smaller becomes the inductor current. At some stage, the inductor current becomes smaller than the load current and from that time onwards the voltage at the capacitor sinks.

In order to avoid that the voltage sinks too far, the switch has to be switched on again, so that the inductor current rises again. However, at the time of switching-on, the inductor current is still lower than the load current, so that the voltage at the capacitor first decreases further. With a conductive switch, the inductor current increases again and becomes at some time larger than the load current. From that time onwards, the voltage at the capacitor increases again. If the current through the inductor becomes too large, the switch is closed again and the current through the inductor becomes smaller again. Now, the cycle starts again from the beginning.

Thereby, the full input voltage is applied to the inductor 100, whereby current starts flowing through the inductor 100, which also charges the capacitor 102. If the resulting voltage at the capacitor is high enough, the control 111 will operate the change-over switch 109, such that it is no longer connected to the positive output rail 114 but to the negative output rail 116. This leads to the fact that the transistor 106 is non-conductive and that no current flows from the source 10, 12 into the network of coil, diode and capacitor. This leads to the fact that the coil operates as generator, in that the potential at the node 103 falls and the coil dispenses a current which continues to charge the capacitor 102 across the diode 104, so that after switching off the switch, the voltage U_(SR) rises further. This increase is decelerated by the current flowing across the load, and turns then into a voltage drop when the current drawn through the load becomes smaller than the current supplied by the coil 100 to the node 114. This voltage drop becomes larger, since the coil does no longer provide enough current. Before the voltage becomes too small, the control acts again and connects the change-over switch 109 again to the positive output rail, which leads to the fact that the switch 106 is closed again and supplies current. Due to the above discussed connections, the output voltage U_(SR) decreases further after switching on, until the positive node 114 of the capacitor has “followed” and the voltage U_(SR) rises again. At some time, the control will operate the change-over switch 109 again, so that the same is connected to the negative output rail 116, so that the transistor 107 is non-conductive, which again causes that the voltage rises further, due to the energy stored in the coil (and the capacitor), which leads to a current across the diode 104 and in the positive node 114.

In this embodiment, the switch 106 shown in FIG. 2 a is designed as normally-off NMOS transistor. NMOS transistors are conductive when the voltage between the gate, which means the control terminal 107 and the source, is higher than a possibly present threshold voltage. Normally-on NMOS transistors already conduct at a voltage U_(GS)>0. In the normally-off NMOS transistor shown in FIG. 2 a, the voltage between the gate 107 and the source (node 118 in FIG. 2 a) has to be higher than the negative of the amount of the threshold voltage U_(th) of the transistor 106. For that reason, the coupling means 113 of FIG. 1 comprises the elements series resistor 113 c, parallel capacitor 113 b and Zener diode 113 a shown in FIG. 2 a.

The Zener diode 113 a is configured such that it operates as voltage source from a certain breakdown voltage onwards, which is configured in a fixed way, which means, in other words, that the breakdown voltage U_(Z) determined by the Zener diode always falls at the parallel circuit of capacitor 113 b and diode 113 a. If the change-over switch 109 is connected to the positive rail 114, the voltage U_(Z) falling at the Zener diode is always applied directly between the gate and source of the transistor. The control input 107 of the transistor is thus coupled to the positive output rail across the parallel circuit of capacitor and Zener diode, in that the transistor becomes conductive.

The only constraint therefore is that the breakdown voltage determined by the Zener diode is higher than the threshold voltage of the transistor. However, this prerequisite can easily be fulfilled, since threshold voltages of transistors are typically not very high and Zener diodes with different defined breakdown voltages exist. Further, the Zener diode only has to be adjusted very loosely to the transistor, since the value of the breakdown voltage of the Zener diode has to be only higher than the threshold voltage U_(th) of the transistor.

Since the requirements for the connection between threshold voltage of the transistor and breakdown voltage of the diode are very loose, deviations of the available transistors can be accepted without a circuit becoming rejected. Thus, the illustrated concept is particularly advantageous in that the rejection rate and thus the costs of the final product can be kept low.

If the control determines that the switch 105 is to be switched off again, it controls the change-over switch 109 such that the same is connected to the negative output rail. This leads to the fact that the potential at the gate 107 of the transistor becomes smaller by U_(SR) than in the switched on state. Thereby, the transistor is non-conductive.

It is a particular advantage of the circuit shown in FIG. 2 a that defined starting of the circuit is ensured. For the defined starting it is particularly unimportant at first, whether the change-over switch 109 is connected to the positive output rail 114 or the negative output rail 116 at switch-on time. Particularly when the change-over switch 109 is realized as multiplexer of transistors or as inverter, it is not defined whether the change-over switch 109 is connected to the upper rail 114 or the lower rail 116, when it is assumed that the whole circuit shown in FIG. 2 has been in a voltage free state prior to switching on, so that all potentials in the circuit are at the value 0.

This starting state, where all potentials are equal 0, will be assumed below, to represent a starting process of the circuit in FIG. 2. Since all potentials equal 0, the potential between gate 107 and source 118 of the switch also equals 0. Since the transistor is a normally-off transistor, the switch is closed at first. If a positive voltage 10, 12 is applied between the first input node 110 and the second input node 112, which is typically the ground node, then first the capacitor C_(Z) 113 b will be charged across the series resistor R_(Z), until the voltage at the capacitor C_(Z) 113 b and the Z diode D_(Z) parallel thereto reaches the threshold voltage U_(th) of the transistor switch 106. As has been discussed, it is unimportant whether the change-over switch 109 is connected to the node 114 or the node 116.

If the change-over switch 109 is connected to the node 114, the voltage U_(Z) will drop directly between the gate 107 and the source 118 of the transistor.

However, if the change-over switch 109 is connected to the negative rail 116, the capacitor 113 b also charges across the series resistor R_(Z) The potential at the node 118 determining the source potential of the transistor is at first not changed from its 0 value, since no charging of the capacitor 102, by which the potential 118 could be risen, takes place, since the change-over switch 109 is connected to the negative rail 116.

Thus, in both instances of the switch 109, the gate source voltage increases from a value of 0 at the time of switching on the source 10, 12 by operating the switch 14 in FIG. 1 to a value equal to the threshold voltage of the transistor. As soon as this is the case, the drain source path of this switch 106 becomes conductive, and the capacitor C_(S) is charged (independent of the position of the switch 109). The charging current for the capacitor 102 (C_(S)) flows at the same time across the coil 100 to ground 112. This leads immediately to the fact that the output voltage U_(SR) increases compared to the potential at the negative output rail 116, due to the increase of the potential at the positive output rail 114. This charging process continues until the voltage at the capacitor C_(S) reaches the voltage at the Z diode minus the threshold voltage of the switch 106.

If the change-over switch 109 is connected to the positive output rail 114, the charging process will be continued, since the transistor 106 is open. If, however, the change-over switch 109 is connected to the negative rail 116, the switch will be closed again, when the potential between gate and source is equal to the threshold voltage. If it is assumed that the potential at the source equals U_(SR), and when it is assumed that the potential at the gate of the transistor is equal to the voltage drop at the Zener diode U_(Z), a value U_(SR) results at the output of the switching regulator, where the transistor is non-conductive again, as a difference between the voltage at the Zener diode and the threshold voltage. If further measures were taken, the transistor would become non-conductive again and the output voltage would remain at U_(SR)=U_(Z)−U_(th).

By dimensioning the Zener diode 113 a this “remaining value” of the output voltage U_(SR) could be chosen freely. In one embodiment, the Zener diode is dimensioned such that U_(SR) becomes so high (when in the “worst case” the change-over switch 109 is connected to the negative rail 116 for startup), that the control 111, preferably supplied by U_(SR), can already operate. Thus, by dimensioning the Zener diode (and the threshold voltage), the voltage U_(SR) is made so high that when this voltage is applied to the control, the states at the nodes in the control are already defined.

Thus, the control will detect a value U_(SR) and compare the same to a limiting value. As soon as the control determines that U_(SR) has obtained the predetermined state during starting up, the control 111 is effective to ensure that the change-over switch 109 is connected to the positive rail 114. If this had already been the case in starting, the control 111 performs no change of the change-over switch 109. If, however, this was not the case, which means the switch 109 as shown in FIG. 2 a was connected to a negative rail 116 at the time of startup, the control will control the change-over switch 109 such that now no longer the negative rail 116 is coupled to the control terminal 107, but that the positive rail 114 is coupled to the control input 107. Thus, the potential of the positive output rail is applied to the anode of the Z diode. Thereby, the voltage at the filter capacitor C_(S) 102 continues to increase due to the charging current flowing-through the charging switch 106, until a desired output voltage is applied to C_(S). Now, the actual voltage regulation follows, by changing over the change-over switch 109 by the control 111 to switch the switch T on or off, respectively, and to thereby regulate the output voltage U_(SR) according to voltage and current requirements of a load (not shown in FIG. 2 a).

Thus, the circuit shown in FIG. 2 a is particularly advantageous in that it starts up in a defined way, wherein it is unimportant at the time of startup in which position the switch 109 is. Further, it is preferred to dimension the control 111 such that it already operates in a defined way when the output voltage is equal to the Zener diode voltage U_(Z) minus the threshold voltage of the transistor 107. Thereby, the startup process can be continued in a defined way without individual supply for the control 111. This leads particularly to an inexpensive circuit, since no specific startup measures have to be taken, except ensuring that the change-over switch 109 is connected to the positive rail 114. Since no specific measures are required and particularly no voltage checks have to be performed with regard to the control 111, the startup process takes place very fast.

Further, it should be noted that the voltage rise can be dimensioned very fast by respective dimensioning of the resistor 113 c and the capacitor 113 b. Thus, it is preferred anyway to dimension the resistor 113 c relatively large, so that the power loss generated by the same does not become significant. Further, it is preferred to dimension the capacitor C_(Z), which anyway only serves for stabilizing the Zener diode 113 a or its (smaller) barrier layer capacitance, also in a small way, so that it is quickly charged to the voltage U_(Z). Thus, starting up takes place without having to consider time constants which would significantly decelerate starting up the switching regulator.

FIG. 3 a shows an alternative embodiment differing from FIG. 2 a in that the transistor T 106 is now a normally-on NMOS transistor and that the coupling means 113 of FIG. 1 realized in FIG. 2 a by elements 113 a, 113 b, 113 c is realized in FIG. 3 a by a simple coupling 113 d. While in FIG. 2 a the potential had been provided with a voltage drop at the first output rail or the second output rail across the coupling means 113, in FIG. 3 a, the potential at the first output rail 114 or the second output rail 116 is directly coupled to the control terminal 107 of the switch 106 by a simple connection. The transistor 106 is designed as normally-on n channel MOS FET or n channel JFET. The configuration of the transistor forming the charging switch 106 is identical to the case of FIG. 2 a with regard to drain and source.

The voltage source or voltage drop means, respectively, from the Z diode with the series resistor and the parallel capacitor of FIG. 2 a, is omitted in FIG. 3 a to achieve a direct potential coupling from an output rail to the control terminal 107 of the switch. Preferably, the threshold voltage of the transistor U_(th) is dimensioned such that it is equal to the Z voltage of the Z diode minus the threshold voltage of the normally-off MOSFET in the first case, so that the output voltage U_(SR) achieves a value in the starting process by which the control 111 can already be operated in order to be able to control the turnover switch 109 at a time of the startup process such that it is connected to the positive rail 114 in a defined way.

Threshold voltages in normally-on NMOS transistors or N JFET transistors are defined in that they define a negative voltage between gate and source of the transistor, where the transistor 106 is just non-conductive. Voltages, which are higher than the negative threshold voltage, cause then that the transistor conducts, while voltages that are more negative than the threshold voltage cause the transistor to turn into a non-conductive state.

In the following, the startup process of the circuit shown in FIG. 3 a will be discussed. Again, it is assumed that all potentials are equal to 0 in the output state. Thus, the switch conducts (it is designed normally-on), which causes the potential at output rail 114 to be raised. It should be noted that at the time of switching on, first both the node 114 and the node 116 are raised to the direct voltage potential, but that the potential difference between the node equals 0. The two nodes 114 and 116 are abruptly raised, since the capacitor, as has been discussed, operates as short at the time of switching-on a direct voltage. Only when charge is provided to the node 114 across the charging switch 106, a potential difference occurs between the node 114 and the node 116, which causes the output voltage U_(SR) to rise from 0 V to values of more than 0 V. Thus, after switching on the voltage, the capacitor 102 is charged across the transistor, which is normally-on.

If the change-over switch 109, which is again designed such that its starting state is undefined, is connected to the positive rail 114, the charging process will be continued, since gate and source of the transistor are shortened, such that U_(GS)=0 V, which always means a conductive state between drain and source in the normally-on transistor. If, however, the change-over switch 109 is connected to the negative rail, the charging state will eventually stop, since the source potential, namely the potential of the positive rail 114, constantly increases. When the source potential is higher than the threshold voltage, the transistor is non-conductive and the output voltage U_(SR) does not raise any further. Thus, as has been discussed, the threshold voltage of the transistor 106 is chosen such that an output voltage U_(SR) applied to the output is already sufficiently high to let the control 111 supplied with the voltage U_(SR) operate in a defined way, so that the same separates the change-over switch 109 from the negative rail 116 and connects the same to the positive rail 114, so that the transistor is opened again, so that the charging process of the capacitor 102 can be continued until the control 111 moves to normal operation and performs a switch control due to a actually desired output voltage U_(SR).

In summary, the start-up process in FIG. 3 a is such that first the switch T conducts. After applying a positive input voltage U_(G1) to the input of the circuit, the capacitor 102 is charged via the switch T and the inductor L, until the voltage at the capacitor C_(S) reaches the threshold voltage of the switch T (when the change-over switch 109 was connected to the negative rail 116). Then the voltage at the output of the circuit U_(SR) is already so high that a controlled operation of the regulator is ensured. The same switches the change-over switch s_(w) such that the positive rail of the output voltage U_(SR) is applied to the gate terminal of the switch. Thereby, the voltage at C_(S) raises further, until the desired output voltage is applied to C_(S). Now, the actual voltage regulation follows. By changing over the change-over switch s_(w) through the regulator circuit 111, the switch T is switched on and off, whereby the output voltage U_(SR) is regulated.

With regard to dimensioning the threshold voltage of the transmitter 106 in the case of FIG. 3 a or the threshold voltage of the transistor 106 and the Zener diode voltage U_(Z) of FIG. 2, respectively, it should be noted that these values are dimensioned such that the maximum output voltage value U_(SR), which is obtained automatically without manipulating the circuit and with predetermined position of the change-over switch 109 on the negative rail 116, which results by itself without any intervention, is already so high that the control can be supplied with U_(SR).

If the control 111 or the threshold voltage, respectively, or the threshold voltage and the Zener diode voltage, respectively, are dimensioned such that the output voltage U_(SR), which is obtained “automatically” is already slightly higher than the voltage where the control 111 operates in a defined way, the control 111 can also be designed to turn the change-over switch 109 up, which means to the positive output rail, immediately, when the same can operate in a defined way, which means when the input voltage is sufficiently high. Thus, the change-over time is not limited to the fact that always the maximum automatically obtainable output voltage has to be present to change-over the switch. Instead, it is required that at some stage in the startup process, it is ensured that the change-over switch is connected to the positive output rail 114.

If, however, a control were used, which operates independent of U_(SR), it could be ensured from the beginning, which means already, for example, at the time of switching on the direct voltage U_(G1), that the switch 109 is connected to the positive rail 114. Due to the simplicity of the circuit, however, it is preferred that the control 111 is supplied by the output voltage U_(SR), wherein in that case the dimensioning of the transistor and possibly the diode is pushed to a limit, in that, for example, 90% of the maximally settable output voltage U_(SR) are used up to change over the change-over switch 109, so that the requirements for the control become as low as possible, since typically circuits that can already be used with low voltages are not compatible with the other voltage levels of the circuits shown in FIGS. 2 a and 3 a, so that an output voltage U_(SR) is preferred, which is as high as possible and settles automatically.

In the embodiments shown in FIGS. 2 a and 3 a, the coil is not coupled to the cathode of the diode as in FIG. 4, but to the anode. This has the advantage that when the change-over switch 109 is connected to the positive rail 114, the potential generated between gate and source of the transistor is not effected by the transient characteristic of the coil. In another embodiment, the coil can also be coupled to the diode on the cathode side, such that the change-over means 109, when it connects the positive rail to the transistor gate 109, either couples or shortens the coil directly to the gate of the transistor, or indirectly couples the same via a voltage drop means.

Disposing the coil on the anode side of the diode has further the advantage that the input voltage of the circuit is only limited by the electric strength of the three elements switch, diode and inductor. Further, the switching regulator supplies itself and starts up in a defined way. Jumping of the common-mode potential of the output voltage between the positive and the negative rail of the input voltage depending on the state of the switch T is of no primary importance for the load, since it does not notice anything of this jumping, since the load merely experiences the potential difference between the upper output rail and the lower output rail, but not the “absolute” potential of the positive output rail or the negative output rail itself.

As has already been discussed, the starting time itself is defined by the dimensioning of R_(Z) and C_(Z), the inductance of inductor L and the capacitance of the filter capacitor C_(S). An additional starting delay for ensuring the voltage supply of the regulator is not required.

Further, it should be noted that by the inventive concept, the input voltage range, which is normally limited to an input voltage ratio of U_(E,max)/U_(E,min)≦5:1, can be increased according to the invention to at least 50:1, as long as the electric strength of the diode, the coil and the switch is correspondingly entrained, since those elements are provided by the input voltage. However, neither the capacitor nor the change-over switch nor the control are provided with such high voltages, so that the maximum input voltage does not have to be considered for these elements, which again results in an inexpensive and at the same time more flexible switching regulator concept according to the present invention.

While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention. 

1. A switching regulator for generating a regulated output voltage, comprising: a first output rail; a second output rail; wherein one output rail can be brought to a negative potential and the other output rail to a positive potential; a diode connected to an output rail; a charging switch connected in series to the diode, wherein the charging switch has a control input by which the charging switch can be opened or closed; a capacitor, wherein the regulated output voltage can be tapped at the capacitor; an inductance coupled to the diode; a change-over switch controllable to couple the first output rail or the second output rail to the control input of the charging switch in dependence on a change-over control signal, wherein the charging switch is formed to be closed due to a potential at one output rail and to be opened due to a potential at the other output rail; and a control for providing the change-over control signal to close the charging switch, so that the capacitor is charged, and to open the charging switch, so that the capacitor is discharged, so that the output voltage can be regulated.
 2. The switching regulator according to claim 1, wherein the charging switch is formed as transistor, wherein a control input of the transistor represents the control input of the charging switch.
 3. The switching regulator according to claim 1, wherein the charging switch is formed as normally-on transistor, which can be brought to a non-conductive state starting from a threshold voltage between a gate terminal and a source terminal, wherein the threshold voltage is set such that in an operating state of the switching regulator, a potential of one of the two output rails is higher than the threshold voltage to switch the transistor into a conductive state, and that a potential of the other of the two output rails is smaller than the threshold voltage to bring the transistor into a non-conductive state.
 4. The switching regulator according to claim 1, wherein the transistor is a normally-off transistor with a threshold voltage, and wherein the change-over switch has a generator for generating a voltage drop, which is, on the one hand, coupled to the gate terminal of the normally-off transistor, and which can, on the other hand, be coupled to the first or second output rail depending on the change-over control signal, wherein the generator for generating a voltage drop is dimensioned such that the amount of the voltage drop is so much higher than the threshold voltage that the transistor becomes conductive when the change-over switch is coupled to an output rail, and to bring the transistor into a non-conductive state when the change-over switch is coupled to the other output rail.
 5. The switching regulator according to claim 4, wherein the generator for generating a voltage drop has a diode, whose breakdown voltage defines the voltage drop.
 6. The switching regulator according to claim 5, wherein the diode is a Zener diode.
 7. The switching regulator according to claim 5, wherein a stabilizing capacitor is connected in parallel to the diode.
 8. The switching regulator according to claim 4, wherein the generator for generating a voltage drop has a series resistor, which is connected between a drain terminal of the transistor and the control input of the transistor, and which is a current source of a transistor and a resistor or another bias source.
 9. The switching regulator according to claim 1, wherein an input voltage can be applied between a terminal of the charging switch, which is not connected to the diode, and a terminal of the diode, which is not connected to the charging switch.
 10. The switching regulator according to claim 1, wherein an output rail, which is connected to a cathode of the diode, is a positive output rail, wherein an output rail, which is coupled to an anode of the diode via the inductance, is a negative output rail, so that the output voltage has a common mode potential differing from a common mode potential of an input voltage.
 11. The switching regulator according to claim 10, wherein the inductance is coupled to the negative output rail via a shunt resistor.
 12. The switching regulator according to claim 1, wherein the positive output rail is ohmically coupled to the terminal of the charging switch, which is connected to the diode.
 13. The switching regulator according to claim 1, wherein the control is formed to be supplied with voltage by the output voltage.
 14. The switching regulator according to claim 1, wherein the control has a startup control, which is formed to control the change-over switch such that, at the latest when a voltage at the capacitor reaches a predetermined value, the change-over switch is controlled in a defined way to get connected to an output rail.
 15. The switching regulator according to claim 14, wherein the change-over switch has a generator for generating a voltage drop, wherein the control is formed to be supplied by the output voltage, wherein the switch is a normally-off transistor with a threshold voltage, and wherein the voltage drop and the threshold voltage are dimensioned such that a difference between the voltage drop and the threshold voltage is sufficient such that the control can actuate the change-over switch in a determined way.
 16. The switching regulator according to claim 1, wherein the change-over switch comprises a direct coupling between the change-over switch and the control terminal of the charging switch, wherein the charging switch is a normally-on transistor with a threshold voltage, wherein the control is supplied by the output voltage, and wherein the threshold voltage of the transistor is dimensioned such that at a threshold voltage of the transistor as supply voltage, the control can connect the change-over switch in a defined way to the output rail, which is coupled to a cathode of the diode.
 17. The switching regulator according to claim 1, wherein the control is formed to be supplied by the output voltage, and wherein the control is formed to control the change-over switch when the output voltage reaches a level where the control can control the change-over switch in a determined way, such that the control input of the charging switch is coupled to the output rail, which is coupled to a cathode of the diode.
 18. A method for controlling starting up of a switching regulator with a first output rail; a second output rail, wherein one output rail can be brought to a negative potential and the other output rail to a positive potential; a diode connected to an output rail; a charging switch connected in series to the diode, wherein the charging switch has a control input by which the charging switch can be opened or closed; a capacitor, wherein the regulated output voltage can be tapped at the capacitor; an inductance coupled to the diode; and a change-over switch controllable to couple the first output rail or the second output rail to the control input of the charging switch in dependence on a change-over control signal, wherein the charging switch is formed to be closed due to a potential at one output rail and to be opened due to a potential at the other output rail, and a control for providing the change-over control signal to close the charging switch, so that the capacitor is charged, and to open the charging switch, so that the capacitor is to be discharged, comprising the steps of: applying a direct voltage between an anode of the diode and a terminal of the charging switch, which is not connected to a cathode of the diode; supplying the control with the output voltage between the first output rail and the second output rail; and controlling the change-over switch by the control, when the output voltage has reached a value where the control can operate in an determined way, to connect the change-over switch to the output rail, which is coupled to the cathode of the diode. 